No, you've missed out your 3cos(x) term.
FYI has no real solutions. This is because for all real x. Adding 1 will always result in a positive answer where x is real
The original question is
What Grandad said is to change - the 3cos(x) term is unaffacted.
I have used square brackets to show this more effectively
You can simplify the constant terms to get it into the form :
You can the solve the quadratic using your favourite method although this one factorises.